1. Field of the Invention
The present invention generally relates to a structure and method for joining panel sheets, and more particularly to such structure and method for joining Zn-plated steel panel sheets with non-plated steel panel sheets.
2. Description of the Related Art
Conventionally, at least one surface (outer surface) of a steel plate (base member or material) is plated with Zn or its alloy in order to prevent corrosion and maintain decent appearance when the steel plate is used as a panel sheet for an automobile body. A common Zn-plated steel panel sheet for an automobile is manufactured by hot or cold rolling a steel plate, and electroplating or hot dipping the steel plate to form a Zn or Zn-alloy plating (collectively referred to as "Zn plating") over a surface of the steel plate.
Referring to FIG. 9A of the accompanying drawings, illustrated is one example of automobile panel sheet 91a that incorporates a steel plate coated with Zn plating. The base member (steel plate) is first plated with Zn or its alloy 95 to provide a Zn-plated steel panel sheet (corrosion-proof steel plate) 94. The steel plate 94 is then subjected to chemical conversion coating treatment with phosphate liquid to create a chemical conversion coating (Hopeite) 96 over the Zn coating. On the phosphate coating 96, a first coating or undercoating 97 is formed by electrodeposition to provide an inner plate 92a. An intermediate coat 98 is further formed over the undercoat 97, and a finish coat 99 is then formed over the intermediate coat 98 to provide an outer plate 93a. This outer plate 93a is the panel sheet 91a. It should be noted that the steel plate 94 has the Zn coating 95 on both surfaces, but only one surface is illustrated.
Referring to FIG. 9B of the accompanying drawings, illustrated is another example of automobile panel sheet 91b that contains a steel plate without Zn plating. A non-plated steel panel sheet 102 is first coated with a chemical conversion coat (Phosphophyllite) 103 by phosphate liquid. Then, an undercoat 97 is applied over the phosphate crystal coat 103 by electrodeposition to provide an inner plate 92b. An outer plate 93b is obtained by further coating the inner plate 92b with an intermediate coat 98 and finish coat 99. The outer plate 93b is the panel member 91b.
It should be noted that an appropriate kind of intermediate coat 98 is selected according to, for example, the type of the automobile. Therefore, it should also be noted that the intermediate coat 98 may be dispensed with if unnecessary.
In general, an automobile panel member is made from a plurality of panel sheets. If an entire panel member of an automobile body is made from a plurality of Zn-plated steel panel sheets 94 only, a cost is raised considerably. Therefore, as illustrated in FIG. 8A of the accompanying drawings, the expensive Zn-plated steel panel sheet is connected to the inexpensive non-plated steel panel sheet in a partly overlapping manner such that the Zn-plated steel panel sheet is located at an area of the body which requires high resistance to corrosion. FIG. 10 of the accompanying drawings illustrates an overlapping portion of the two sheets in an enlarged scale.
Referring now to FIGS. 11A and 11B of the accompanying drawings, a chemical conversion coating (Hopeite) 96 is formed over the surface of a Zn plating 95a by phosphoric acid etching, and another chemical conversion coating (Phosphophyllite) 103 is formed over the surface of the non-plated steel panel sheet 102 ("Normal Area" in FIG. 10) by phosphoric acid etching. Anode and cathode exist microscopically (they are present very closely), and therefore an anode reaction (phosphoric acid etching) and a cathode reaction (generation of chemical conversion crystal around the etched area) take place on the same steel plate. As a result, the crystal precipitates and grows to extend over the anode-etched surface which is fresh and reactive (zones designated at 101 in FIG. 10). Thus, the Zn plating 95a and chemical conversion coating (Hopeite) 96 are firmly joined with each other, and the non-plated steel panel sheet 102 and chemical conversion coating (Phosphophyllite) 103 are also firmly joined with each other.
As depicted in FIG. 10, however, the panel member always has at least one connection of the Zn-plated steel panel sheet 94 and non-plated steel panel sheet 102. In the vicinity of the connection, a voltage difference is produced due to contact of different metals when dipped in the chemical conversion treatment liquid.
Referring to FIGS. 12A and 12B, consequently, Zn (base metal) becomes an anode in the surface of the non-plated steel panel sheet 102 near the two-sheet connection on the connection side ("Weak Adhesion Area" in FIG. 10). Therefore, Zn is only dissolved by the anode reaction, and crystal precipitates and grows on the non-etched steel surface. In other words, although a chemical conversion crystal is formed on the Fe-based steel plate in the range of about 20 mm from the two-sheet connection, it is not the chemical conversion coating (Phophphyllite) 103 but the chemical conversion coating (Hopeite) 96 having no Fe.
The chemical conversion coating (Hopeite) 96 is difficult to adhere onto the non-plated steel panel sheet 102 and easy to exfoliate. Therefore, connection of the Zn-plated steel panel sheet 94 and non-plated steel panel sheet 102 (different metal connection) is not applicable to those areas of a vehicle body panel which are subjected to an external stress. Such areas of the vehicle body are, for example:
(1) Front and roof panels of a vehicle body, which often experience pitching movements; PA1 (2) Panels near vehicle doors, which frequently experience vibrations and frictions due to opening and closing; PA1 (3) Panels of which two-sheet connection is often exposed to water or moisture due to an inherent structure of the vehicle (areas where edge corrosion tends to occur); PA1 (4) Panels which a service man's legs, hands and body tend to touch or hit; and PA1 (5) Panels onto which optional parts such as ski carriers are mounted.
Therefore, even if the non-plated steel panel sheet 102 itself possesses a corrosion resistance required by the vehicle body, it cannot be used for those areas such as mentioned above. Instead, the expensive Zn-plated steel panel sheets 94 should be used. The Zn-plated steel panel sheets are joined with each other to provide a panel member for such areas because the same metal connection does not produce a voltage difference in the chemical conversion treatment. Accordingly, a Zn-plated steel panel sheet utilization ratio is high in the conventional body panel.